It is known for engines, especially diesel engines designed for heavy duty applications, to be fitted with a compression braking system. The compression braking system allows large amounts of energy to be dissipated by the engine by introducing an additional exhaust valve opening close to Top Dead Centre (TDC) of the compression stroke such that the compressed gas is released into the exhaust system. The engine is therefore operating as an air pump and no fuel is added during this mode of its operation. Often a further exhaust valve opening occurs during the intake stroke when the compression brake is operated in order to reduce intake pumping losses.
Methods for producing an additional exhaust event that is selectable in order to allow a compression braking mode of operation are well known in the prior art (e.g. U.S. Pat. No. 3,220,392). Typically conventional compression brake systems are either on or off, and as a result there is no facility for changing the amount of energy that is dissipated by each cylinder. Some engines do however operate a compression brake system on different groups of cylinders in order to provide some control of the braking effort.
An alternative method for changing the effect of a compression brake would be to change the timing of the additional exhaust valve opening. Advancing the timing of the additional valve opening to a position some way before TDC will release the gas from the cylinder before it has been fully compressed, and this will reduce the braking effect. Reducing the braking effect will also reduce the noise generated as the compressed gas is released from the cylinder—the use of compression brakes is banned in some areas at night due to the noise they make.
There are also a number of engine combustion strategies that have been proposed which use additional valve openings, for example an additional exhaust opening in the intake stroke may be used to generate internal EGR (e.g. US2006102121).
As a secondary valve opening to modify the combustion process and a compression brake are never required at the same time, it would be advantageous to use a single system to produce both types of secondary valve lift.